This application is based on Japanese patent applications No. 10-135410 filed on May 18, 1998, and No. 10-135411 filed on May 18, 1998, the entire contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to a solid-state image pickup device, more particularly to a solid-state image pickup device having a dual-line type horizontal charge transfer device.
b) Description of the Related Art
FIG. 2 is a diagram showing the structure of a solid-state image pickup device 1 having a dual-line type horizontal charge transfer device 5.
A pixel array 2 comprises a plurality of photo diodes (photoelectric converter elements) 3 arranged in a flat matrix form, and a plurality of vertical charge transfer devices 4. The photo diodes 3 convert received lights into charges, and each of the photo diodes 3 corresponds to one of the pixels which form a two-dimensional image. The photo diodes 3 transfer the charges to the plurality of vertical charge transfer devices 4 which transfer the charges in the vertical direction.
The dual-line type horizontal charge transfer device 5 comprises a first horizontal charge transfer device 5a and a second horizontal charge transfer device 5b. The vertical charge transfer devices 4 and the horizontal charge transfer devices 5a and 5b comprise charge coupled devices (CCD). The charges in the vertical charge transfer devices 4 are transferred downward in the vertical direction toward one of the first horizontal charge transfer device 5a or the second horizontal charge transfer device 5b. The first and second horizontal charge transfer devices 5a and 5b horizontally transfer the charges leftward.
The first horizontal charge transfer device 5a transfers the charges to a first amplifier 6a. The first amplifier 6a amplifies the received charges and outputs the amplified charges. The second horizontal charge transfer device 5b transfers the charges to a second amplifier 6b. The second amplifier 6b amplifies the received charges and output the amplified charges.
A solid-state image pickup device used in, for example, a high definition television (HDTV), which is designed for high resolution image capturing is required to transfer charges quickly because it has a large number of photo diodes (pixels). In such a case, the solid-state image pickup device 1 having the dual-line type horizontal charge transfer device 5 is used in order to improve the charge transfer efficiency in the CCD and equalize amplification sensitivity.
The structure of a boundary area 7 between the first and second horizontal charge transfer devices 5a and 5b will now be described.
An upper diagram of FIG. 3 is a plan view showing the boundary area 7. The portrait direction of FIG. 3 corresponds to the landscape direction of FIG. 2. A shift gate 12 is disposed between the first and second horizontal charge transfer devices 5a and 5b. Graphs shown in middle and lower sections of FIG. 3 show potential energy variation in the boundary area 7 wherein the horizontal axes indicate positions in the boundary area 7 and the vertical axes indicate the potential energy variation against the charges (electrons).
Potential energy waveform S1 shown in the middle graph of FIG. 3 represents potential energy variation when the shift gate 12 is closed because a gate signal is not applied thereto. Since the shift gate 12 is closed, the charges in the first horizontal charge transfer device 5a which are transferred from the vertical charge transfer devices 4 (FIG. 2) stay in the first horizontal charge transfer device 5a. 
Potential energy waveform S2 shown in the lower graph of FIG. 3 represents potential energy variation when the shift gate 12 is open after the gate signal is applied thereto. When the gate signal is applied to the shift gate 12, potential energy of the second horizontal charge transfer device 5b decreases because the second horizontal charge transfer device 5b is biased. Since the shift gate 12 is open, the charges in the first horizontal charge transfer device 5a which are transferred from the vertical charge transfer devices 4 (FIG. 2) are further transferred in the vertical direction (the landscape direction in FIG. 3) toward the second horizontal charge transfer device 5b. The transferred charges 13 will stay in the second horizontal charge transfer device 5b after the shift gate 12 is closed.
That is, the charges in the vertical charge transfer devices are controlled so as to be transferred one of the first and second horizontal charge transfer device 5a and 5b by switching the shift gate 12.
An upper diagram in FIG. 4 is a plan view for explaining a transfer operation of the charges 11 in the first horizontal charge transfer device 5a. 
The horizontal charge transfer device 5a comprises groups each consisting of a first well region W1, a first barrier region B1, a second well region W2 and a second barrier region B2. A predetermined number of the groups are arranged in the horizontal direction. A drive signal Hxcfx861 is applied to the first well region W1 and the first barrier region B1. A drive signal Hxcfx862 is applied to the second well region W2 and the second barrier region B2. That is, the horizontal charge transfer device 5a is driven by the dual-phase drive signals Hxcfx861 and Hxcfx862.
Graphs shown in middle and lower sections of FIG. 4 represent the potential energy variation in the horizontal charge transfer device 5a wherein the horizontal axis indicates positions in the horizontal charge transfer device 5a and the vertical axis indicates the potential energy variation.
Potential energy waveform S1 shown in the middle graph of FIG. 4 represents potential energy variation when the drive signals Hxcfx861 and Hxcfx862 are 0V. Effective dopant concentration is adjusted so that the potential energy of the well regions W1 and W2 is lower than that of the barrier regions B1 and B2. For example, the well regions W1 and W2 are n-type regions having high dopant concentration and the barrier regions B1 and B2 are n-type regions having low dopant concentration. The well regions W1 and W2 show almost the same potential energy level. The barrier regions B1 and B2 also show almost the same potential energy level.
Potential energy waveform S2 shown in the lower graph of FIG. 4 represents potential energy variation when the drive signal Hxcfx861 is 5V while the drive signal Hxcfx862 is 0V. According to the graph, potential energy gradient appears in the horizontal charge transfer device 5a. That is, the potential energy level gradually decreases from higher potential energy region B2 to lower potential energy region W1. In the horizontal charge transfer device 5a, the charges 11 are transferred leftward in the horizontal direction in accordance with the potential gradient.
FIGS. 5A to 5D are cross sectional views showing a device for explaining steps of manufacturing a horizontal charge transfer device (charge coupled device) in the prior art.
As shown in FIG. 5A, n-type dopant 23 is added to a p-type silicon region 21 of a silicon substrate by ion implantation. As a result, an n-type silicon region 22 is formed on the p-type silicon region 21.
Then, a silicon oxide layer 24 is formed on the n-type silicon region 22 as shown in FIG. 5B, and then patterned first poly gates 25 made of amorphous silicon are formed on the silicon oxide layer 24. The first poly gates 25 work as electrodes for the well regions W1 and W2.
Then, p-type dopant 27 is added to the substrate by ion implantation as shown in FIG. 5C. During the doping, the first poly gates 25 work as a mask. As a result, p-type silicon regions 26 are formed on exposed surfaces of the n-type silicon regions 22 which are unmasked by the first poly gates 25. The p-type silicon regions 26 and the n-type silicon regions 22 underneath correspond to the barrier regions B1 and B2 respectively. The n-type silicon regions 22 under the first poly gates 25 correspond to the well regions W1 and W2.
As shown in an upper diagram of FIG. 5D, the silicon oxide layer is subjected to anisotropy etching so as to be etched partially. During the etching, the first poly gates 25 work as a mask. Then, a silicon oxide layer 28 is deposited onto the whole surface of the substrate by thermal oxidization and/or CVD. And then, patterned second poly gates 29 made of amorphous silicon are formed on the silicon oxide layer 28. The first poly gates 25 work as gate electrodes for controlling the n-type silicon regions (well regions) 22 beneath, and the second poly gates 29 work as gate electrodes for controlling the p-type silicon regions (barrier regions) 26 thereunder and the n-type silicon regions 22 under the p-type silicon regions 26.
A lower diagram of FIG. 5D is a graph showing potential energy waveform S1 which represents potential energy variation in the n-type regions 22 when a voltage applied to the first and second poly gates 25 and 29 are 0V. The p-type silicon regions 26 under the second poly gates 29 and the n-type silicon regions 22 under the p-type silicon regions 26 are barrier regions B1 and B2 because their potential energy level is high. On the contrary, the n-type silicon regions 22 under the first poly gates 25 are well regions W1 and W2 because their potential energy level is low.
The first and second horizontal charge transfer devices can transfer the charges in the horizontal direction in accordance with the applied drive signals Hxcfx861 and Hxcfx862. In case of the dual-line type horizontal charge transfer device 5, however, it is difficult to realize smooth charge transfer in the vertical direction from the first horizontal charge transfer device 5a to the second horizontal charge transfer device 5b in accordance with the controlled shift gate 12 without affecting the horizontal charge transfer.
A dual-line type horizontal charge transfer device 5 which is designed to resolve the above problem will now be described.
FIG. 6A shows the structure of the boundary area 7 (FIG. 2) in the prior art. The shift gate 12 is disposed between the first and second horizontal charge transfer devices 5a and 5b. The order of the four regions shown in FIG. 6A differs from that of the four regions shown in FIG. 4 for the sake of explanation convenience. That is, the group of the four regions shown in FIG. 6A starts with the region W2 (the leftmost region) which is two regions behind the starting region of the region group shown in FIG. 4.
Each of the horizontal charge transfer devices 5a and 5b comprises the groups of the regions which are arranged horizontally. Each of the groups comprises the second well region W2, the second barrier region B2, the first well region W1 and the first barrier region B1. The drive signal Hxcfx861 is applied to the first well region W1 and the first barrier region B1, and the drive signal Hxcfx862 is applied to the second well region W2 and the second barrier region B2. The horizontal charge transfer devices 5a and 5b are driven by the dual-phase drive signals Hxcfx861 and Hxcfx862.
The well regions W1, W2 and the barrier regions B1, B2 on the first horizontal charge transfer device 5a are tapered. In the well regions, each one end near the second horizontal charge transfer device 5b is broader than another end. In the barrier regions B1 and B2, on the contrary, each one end near the second horizontal charge transfer device 5d is narrower than another end.
In FIG. 6B, broken lines represent electric field appeared in the first well region W1. Since the well region W1 has the tapered shape, a side effect causes the broader end portion of the well region W1 to have a higher voltage (lower potential energy) than the narrower end. And built-in potentials caused by the differences in the dopant concentration appear around boundaries between the well region W1 and adjacent regions, that is, the barrier regions B1 and B2. Potential energies around the boundaries are lower than those around the center of the well region W1. Accordingly, the tapered well region W1 causes smooth potential gradient in the vertical direction.
As indicated by the potential energy waveform S2 shown in FIG. 4, the charges 11 are horizontally transferred to the first well region W1. And then, the charges 31 in the well region W1 are transferred downward in the vertical direction as shown in FIG. 6A in accordance with the potential gradient. In response to opening the shift gate 12, the charges 31 are transferred to the second horizontal charge transfer device 5b through channel stop regions 33a and 33b. Dopant whose conductance is reversed to that of the well regions W1 and W2 is added to the channel stop regions 33a and 33b. The potential energy of the channel stop regions 33a and 33b is higher than that of the well regions W1, W2 and the barrier regions B1, B2.
FIG. 16 is a cross-sectional view showing the well region W1 and the barrier region B2 along a line Axe2x80x94A shown in FIG. 6A. As shown in FIG. 16, the shift gate 12, the well region gate electrodes 25 and the barrier region gate electrodes 29 are formed on the silicon oxide layer 24 which cover the well regions W1 and the barrier regions B2. The shift gate 12 and the gate electrodes 25 and 29 are insulated from each other via the silicon oxide layer 24. The shift gate 12 is formed so as to be placed just above the boundary between the well region W1 and the barrier region B2. The shift gate 12 works as an electrode to shift the charges from the first horizontal charge transfer device 5a to the second horizontal charge transfer device 5b. 
The method of forming the well region gate electrodes 25 and the barrier region gate electrodes 29 is aforementioned with reference to FIGS. 5A to 5D. Voltages are applied to the well region W1 and the barrier region B2 through the well region gate electrode 25 and the barrier region gate electrode 29.
Since the well region W1 is tapered, smooth potential energy gradient appears, thus, the charges are smoothly transferred in the vertical direction. However, it is difficult to obtain large potential gradient because available potential gradient is limited.
It is an object of the present invention to provide a charge transfer device which can smoothly transfer charges from a first charge transfer device to a second charge transfer device.
It is another object of the present invention to provide a method of manufacturing a charge coupled device which is adaptable to a solid-state image pickup device having a dual-line type horizontal charge transfer device for smoothly transferring charges in the vertical direction.
According to one aspect of the present invention, it is provided a charge transfer device which comprises:
a first charge transfer device including well regions having low potential energy and barrier regions having high potential energy, which can transfer charges supplied thereto from an external device;
a second charge transfer device which can transfer charges supplied thereto from said external device via said first charge transfer device; and
a shift gate between said first and second charge transfer devices, which control the externally supplied charges to be transferred to said first charge transfer device or said second charge transfer device, wherein
said first charge transfer device is a semiconductor region between said external device and said second charge transfer device; and
each of said well regions of said first charge transfer device comprises a first highly-doped region including a poorly-doped region and a tapered section whose one end near said second charge transfer device is broader than another end near said external device.
According to another aspect of the present invention, it is provided a charge transfer device which comprises vertical charge transfer devices which transfer charges in the vertical direction, first and second horizontal charge transfer devices which transfer the charges from the vertical charge transfer devices in the horizontal direction, and a shift gate which controls the charges from the vertical charge transfer devices to be supplied to one the first horizontal charge device or the second horizontal charge transfer device, wherein the first horizontal charge transfer device is a semiconductor region between the vertical charge transfer devices and the second horizontal charge transfer device and includes highly-doped regions having tapered portions whose one ends near the second horizontal charge transfer device are broader than another ends near the vertical charge transfer devices.
Since the well region in the first horizontal charge transfer device has a tapered highly-doped region whose one end near the second horizontal charge transfer device is broader than another end near the vertical charge transfer devices, a potential energy grade appears. That is, the potential energy near the vertical charge transfer device is higher than that near the second horizontal charge transfer device. Since the charges are smoothly transferred from the first horizontal charge transfer device to the second horizontal charge transfer device, charge transfer efficiency is improved.
According to still another aspect of the present invention, it is provided a method of manufacturing charge coupled devices to be used in a solid-state image pickup device which comprises a first horizontal charge transfer device which horizontally transfers charges supplied from a plurality of vertical charge transfer devices, a second horizontal charge transfer device which horizontally transfers the charges supplied from said plurality of vertical charge transfer devices via said first horizontal charge transfer device, and a shift gate which selectively supplies the charges from said plurality of vertical charge transfer devices to said first horizontal charge transfer device or said second horizontal charge transfer device wherein said first horizontal charge transfer device comprises said charge coupled devices, said method comprises the steps of:(a) forming on a semiconductor substrate first n-type regions to be said first horizontal charge transfer device;(b) forming a first insulation layer on said first n-type regions;(c) forming a patterned first electrode layer on said first insulation layer comprising electrodes for transferring the charges in said first horizontal charge transfer device;(d) doping n-type dopant into said semiconductor substrate by ion implantation while using said first electrode layer as a mask to form well region s on said first horizontal charge transfer device; and (e) doping n-type dopant into predetermined regions on said well regions by ion implantation while using said first electrode layer as a partial mask.
The well regions are self-aligned by ion implantation while using the first electrode layer as a mask. Moreover, highly-doped n-type regions are selfaligned in predetermined regions by doping n-type dopant into the predetermined regions on the well regions by ion implantation while partially using the first electrode layer as a mask. Forming the highly-doped n-type regions so as to have tapered shape helps a dual-line type horizontal charge transfer device consisting of charge coupled devices in a solid-state image pickup device to improve its vertical charge transfer efficiency. The first horizontal charge transfer device can smoothly transfer the charges not only in the horizontal direction but also in the vertical direction toward the second horizontal charge transfer device.